6o

objects will appear to diminish when viewed through it. But if a small lens of this construction be inserted between the speculum and the eye-piece, the convergent rays falling upon it will be rendered more parallel, and therefore give the same result as if a glass of longer focus was employed, consequently the magnifying power will be thereby increased. (See Sir J. Herschel on the Telescope, art. 33).

This lens, when constructed in the usual manner, and placed about 5 inches from any eye-piece, increases the magnifying power by half as much again. It will thus be very easy to double any series of eyepieces that may be chosen. For instance, suppose the series are 70, 130, and 250, these multiplied by 1*5 will give an extra series of 70, 105, 130, 195, 250, and 375. A variation of each of these powers can be obtained by shifting the position of the lens in the draw-tube.

We have lately placed a Barlow’s lens between the flat mirror and the speculum, thereby considerably diminishing the size of the flat and securing a great increase of power and a flat field, without interfering in the least with definition.

THE MICROMETER.

This accessory to the Telescope is useful for many purposes. In the first place, it enables the diameter of any object, or its distance from another, to be accurately measured; and, secondly, when a position circle is applied to it, it determines the angle that a line joining these objects makes with the meridian. One form of this instrument is that called The Parallel Wire Micrometer; it consists of a small box within which slide two frames, on each of which are fixed some fine spider’s threads, and which can be shifted apart by means of screws with graduated heads. Its mode of employment is as follows :—Suppose it be wished to measure the diameter of a planet, a wire properly focussed by the eye-piece would be brought to each side of the disc, and the distance apart of the wires then ascertained by the number of revolutions of the graduated heads that have been necessary. A notched scale of teeth also placed in the focus of the eye-piece assists in the rough measurement, accuracy being obtained by means of the graduated heads.

In this and in all other forms of micrometer the value of the divisions on the graduated head and of its entire revolution must be ascertained before the result just obtained can be reduced to minutes and seconds of arc. This may be done thus :—Separate the wires by any known number of revolutions of the head and allow an equatorial star to pass from one wire to the other. Accurately determine, by means of a clock or watch, the time this takes,' and the amount, reduced to minutes and seconds of arc (page 76), will be the distance between the wires in that position; this can easily be calculated for any position of the threads, and for any number of turns of the heads or fractions of them.

---

Le texte affiché peut comporter un certain nombre d'erreurs. En effet, le mode texte de ce document a été généré de façon automatique par un programme de reconnaissance optique de caractères (OCR). Le taux de reconnaissance estimé pour cette page est de 99,41 %.

La langue de reconnaissance de l'OCR est l'Anglais.

---